Rainforest

Burning issues

A new experiment is setting the Amazon on fire

THIS month Jennifer Balch will head into the Amazon rainforest of Mato Grosso state, in Brazil. She intends to set fire to it and find out what happens. When Dr Balch, who is based at Woods Hole Research Centre, in Massachusetts, and her 30 helpers have finished their weeklong task, 50 hectares will have been torched. “It's pretty darn exciting, and a bit crazy”, she says, “to see a bunch of researchers running around burning down a forest.”

The questions that prompt all this destruction are important. The first is: will tropical forests survive the increasing occurrence of wildfires as the climate changes and people move in, or will the landscape shift from one ruled by trees to one dominated by grassland? The second is: how much carbon do such wildfires release into the atmosphere?

Wildfires are certainly a problem in Brazil. During the droughts of 1997 and 1998, which were linked to El Niño, an irregular warming of the Pacific Ocean, 39,000 square kilometres of Amazonian forest were burned by such fires. That is twice the area deliberately cut down in Brazil each year between 1988 and 2005. Similar fires occurred all over South-East Asia and Latin America, causing $19 billion-24 billion of damage through loss of timber, destruction of property and the effect of the smoke on human health.

Tropical wildfires are different from the ones that happen in temperate woodlands, for example in the western United States. Temperate-forest fires burn hot and fast, and sometimes all the way up to the crown of a tree. Tropical forest is wetter and usually supports only knee-high fires that spread slowly through the understorey. Indeed, they are slow enough to be outrun—which is convenient for Dr Balch and her associates.

Tropical fires are, nevertheless, a significant source of carbon dioxide. And their increasing frequency is an important part of the degradation of the Amazon forest—and could be an indicator of large-scale environmental change. To find out more about what such fires presage, Dr Balch has been conducting experimental burns like the one now planned since 2004.

At the start of each experiment the group cuts a fire break along the perimeter of the area. This means removing all vegetation, right down to the soil. Then researchers fan out across the forest measuring the temperature and the humidity of the air (two of the factors that determine how a fire will behave), and recording the amount of woody fuel in the forest and the moisture in that fuel. Five minutes after that come the fire-raisers, who light 10km of “fireline” using drip torches (devices that drop flaming paraffin at a constant rate). Five minutes after them it is the turn of the unfortunate team charged with measuring the height of the flames and mapping the fireline in order to calculate how fast it is moving—all without inhaling too much smoke as they go.

The results so far are worrying. Though many of the largest trees in the Amazon can survive repeated fires, smaller plants do less well. In those areas that have had three or four periods of experimental burning only half the number of species of tree seedlings originally found are still there. Grasses, by contrast, are doing well. So far, they have invaded as far as 200 metres from the edge of the forest.

That is worrying for those who prefer forest to savannah. Global warming alone is predicted to favour the conversion of woodland to grassland, and Dr Balch thinks her work shows that wildfires may accelerate the process. Grasses are well adapted to fire, being able to resprout quickly from their roots and thus outcompete tree seedlings.

The goal, then, is to encourage landowners to do more to prevent wildfires from starting and spreading. Those landowners, however, are in it for the money. At the moment, standing forest is of little commercial value, and much of it continues to exist only because of laws intended to prevent total clearance of the land. But talk of financial incentives for forest-owners to preserve their trees is in the air, and a deal may be struck at the United Nations climate-change meeting being held in Copenhagen in December.

Intriguingly, Dr Balch's experimental area is part of an 80,000-hectare holding owned by the family of Blairo Maggi, a soyabean magnate who is the governor of Mato Grosso. In 2005 Mr Maggi “won” the Greenpeace “golden chainsaw” award for deforestation. But if money to preserve trees were on the table his views might change. Dr Balch has done a back-of-the-envelope calculation. Assuming a conservative value of $5 per tonne of carbon locked up by trees, she calculates that a wildfire which spread through half of the landholding that Mr Maggi is legally required to keep as forest would cost him $16m. That would make intact rainforest hot property indeed.